Added experimental alpha support to fancy door, disabled by default.

To enable, uncomment "use_texture_alpha" on lines 72 and 99 of
doors_and_gates.lua, and make sure that if you are using a texture pack,
that it has support for alpha on these doors. (HDX does, as do the
default textures).

Note that alpha support is glitchy - without shaders it will not work at all,
and the doors' nodeboxes will be screwed up.  With shaders, it mostly works,
but the whole door is slightly translucent where it should not be.
Made door alpha enabled by default - but you gotta be using shaders if
you are running an alpha-capable client!
Use homedecor_blanktile.png on the top face of bottom half of door and vice
versa.  Gets rid of the visual division between the two halves of the glass
door.
Better selection boxes and inventory textures for shingles and skylights.
Also added frosted glass skylight
Added sounds to doors and gates
reset sound gain to 1.0 for now, since larger values don't work anyway.
Fixed a node replacement bug in doors
rewrote and streamlined the doors code in the process
also fixed missing detection of infinite stacks (e.g. default creative mode)
Got rid of all alternate-color recipes
(e.g. mahogany from coal+dirt).

Rewrite all paint-based recipes to use the equivalent white or grey dyes
instead.

Rewrite a few other recipes to use default dye groups instead of directly
calling on unified dyes colors, wherever possible.

Changed a couple of recipes to use slightly more logical ingredients.
a few more craft tweaks
changed recipes for white glowlights